Process for preparing catalytically active coatings for the synthesis of hydrogen cyanide

ABSTRACT

A process for preparing catalytically active coatings for the synthesis of hydrogen cyanide on moulded items substantially consisting of aluminum oxide. The catalytically active coating contain, as active components, at least one platinum group metal and a nitrate which are applied to the moulded item by means of a coating dispersion which contains the active components as finely divided solids. This means that the hitherto conventional production of aluminum nitride during the forming process is not required. The coatings therefore achieve their final activity level, which is well above that of conventional coatings, very rapidly. In addition, these catalysts can be started up under high load and have a lower operating temperature.

INTRODUCTION AND BACKGROUND

The present invention relates to catalytically active coatings on thesurfaces of moulded shaped bodies for the synthesis of hydrogen cyanidefrom hydrocarbons and ammonia, wherein the catalytically active layercontains nitrides and at least one platinum metal. More particularly,the present invention relates to the process of making these catalysts.In another aspect, the present invention relates to the use of thecatalyst produced as described above for the synthesis of hydrogencyanide from hydrocarbons and methane.

According to the so-called BMA process developed by Degussa A. G., lowerhydrocarbons, in particular methane, are reacted with ammonia attemperatures of about 1000 to 1350° C. in the presence of a catalyst togive hydrogen cyanide (HCN) and hydrogen (see Ullmann's Encyclopedia ofIndustrial Chemistry, 5th edition 1987, vol. A 8, pages 162-163incorporated herein by reference). The strongly endothermic reactiongenerally takes place in tubular reactors. The internal surface of thereaction tubes, substantially made from aluminum oxide, which aresuspended in a BMA tubular reactor and are externally heated, isprovided with a catalytically active coating.

According to the process in DE-A 10 13 636, the reaction gases arepassed over catalysts which, in addition to one or more platinum groupmetals, in particular platinum, also contain aluminum or elements fromthe lanthanide series, individually or several together, substantiallyin the form of their nitrides. A preferred catalytically active coatingon moulded bodies made of aluminum oxide contains platinum and aluminumnitride.

To prepare this type of coating, the moulded bodies are impregnatedwith, for example, aluminum-containing solutions of hexachloroplatinicacid. After drying, the catalytically active components are reduced withhydrogen at 600 to 900° C. The nitride is formed in the BMA reactorduring the start-up phase, under the effect of the ammonia used for HCNsynthesis. The start-up phase, only after which does the catalystachieve its full potential, lasts about 25 hours.

A substantially improved process for preparing the catalytically activecoating is described in DE 39 23 034 C2. According to this patent, thesurfaces of the moulded bodies are wetted with a dispersion whichcontains, as coating components, particulate elemental platinum groupsmetal and particulate elemental aluminum metal with the particle sizesof each being substantially less than 100 μm. The surfaces of themoulded bodies treated in this way are slowly heated up to the reactiontemperature of the BMA process, 1000 to 1350° C., in the presence ofnitrogen and/or ammonia in order to convert the coating into thecatalytically active state thereby forming the catalyst needed for thechemical reaction producing the hydrogen cyanide. If ammonia is usedduring the heating procedure, then the catalyst has been effectivelyformed on achieving the reaction temperature.

An object of the present invention is to provide a process for preparingcatalytically active coatings on the surface of moulded bodiessubstantially made from aluminum oxide, for the synthesis of hydrogencyanide, which provide catalytically active coatings having a highercatalytic activity than conventional coatings and capable of beingloaded in a very short time with the high reactant flow-rates presentduring steady-state operation.

SUMMARY OF THE INVENTION

The above and other objects of the invention are achieved by a processfor preparing catalytically active coatings on the surface of shapedceramic moulded bodies used for the synthesis of hydrogen cyanide fromhydrocarbons and ammonia by uniformly wetting the moulded bodies with acoating dispersion containing at least one particulate elementalplatinum group metal and a further coating component in a carrierliquid, evaporating the carrier liquid and converting the coating intothe catalytically active state by slowly heating to 1000 to 1350° C. inthe presence of nitrogen and/or ammonia.

A feature of the invention is using particulate nitrides of the elementsaluminum, boron, titanium and silicon as cations, individually or as amixture, as the further coating components, wherein the particulatecomponents have average particle sizes of substantially less than 100μm.

The particulate metals and particulate nitrides contained in thedispersion have particle sizes of less than 100 μm, preferably less than50 μm. Coating dispersions with the smallest possible particle sizes arepreferred because the particles in this type of dispersion are lesslikely to settle out.

DETAILED DESCRIPTION OF INVENTION

In carrying out the present invention the coating dispersion contains amember selected from the platinum group metals, that is, rhodium,ruthenium, palladium, osmium, iridium and platinum as well as mixturesthereof. Platinum is preferred. The platinum metals are obtainable infinely divided elemental form in a simple manner by, for example,reducing solutions of their compounds, wherein so-called blacks of theplatinum metals are particularly preferably used in the processaccording to the invention. Platinum blacks are commercially available.

The particulate nitrides that are used in the present invention toinclude in the coating are generally prepared from the relevant elementand ammonia and/or nitrogen (e.g. G. Selvaduray and L. Sheet, Mater.Sci. Technol., 9 (1993), 463-473). The nitrides of Al, B, Ti and Si canbe formed in this way. Other elements, such as e.g. lithium, whichcatalyze the reaction may also be added. The powders obtainable in thisway are commercially available in a variety of qualities and puritiesfrom different manufacturers.

Nitrides of aluminum, boron, titanium and silicon may be used separatelyor as a mixture for the catalytic coating. Aluminum nitride ispreferably used.

Solvents which are suitable as carrier liquids for preparing the coatingdispersion are, for example, inert organic solvents such as aliphaticand aromatic hydrocarbons, esters, ketones or alcohols and mixtures ofthese types of solvent. Solvents or mixtures with a boiling point orboiling range below 350° C., in particular below 150° C., are extremelysuitable. Lower alcohols and aromatic and non-aromatic hydrocarbons areparticularly preferred carrier liquids.

The solids concentration of the dispersion may vary between wide limits,provided the dispersion has the desired processing viscosity. Ingeneral, the dispersions contain 10 to 300 wt. % of metal powder andnitride powder, preferably 30 to 200 and in particular 50 to 150 wt. %,with reference to the carrier liquid.

By adjusting the concentration of metal powder and nitride powder in thedispersion it is possible to apply the amount of platinum metal andnitride required to produce a long operating lifetime for the coatedmoulded body in a single coating step. A concentration of less than 10mg of platinum metal per cm² of catalytically active surface isperfectly adequate. A surface concentration of 0.05 to 5 mg of Pt/cm² ispreferred, in particular of 0.1 to 2 mg/cm².

In the dispersion, the atomic ratio of platinum metals to cations ofnitrides is 0.001 to 1:1. A ratio adjusted to be in the range 0.01 to0.5:1 is preferred. A Pt to cations ratio in the range 0.01 to 0.2 isparticularly appropriate.

After evaporating the carrier liquid from the dispersions, the coatedmoulded bodies are slowly heated to 1000 to 1350° C. in the presence ofnitrogen and/or ammonia. The heating period depends strongly on thefurnace used and the properties of the moulded bodies. The BMA reactiontemperature is generally reached within 2 to 20 hours, mostly 5 to 15hours. After a further treatment time of 2 to 10 hours at the reactiontemperature, forming of the catalyst is complete.

In conventional production processes aluminum nitride, inter alia, isproduced during forming in the temperature range between 600 and 1000°C. In contrast, the formation of nitrides during the forming procedurein the process according to the invention does not need to take place,due to the use of nitrides in the coating dispersion.

The synthesis of hydrogen cyanide can start immediately after theforming process, without interrupting the ammonia supply, by addingmethane. Optionally, the ammonia stream is first adjusted to the valuerequired for steady-state operation. The methane supply is continuouslyincreased until a molar ratio of methane flow to ammonia flow of 0.8 to0.99 is achieved. The methane to ammonia molar ratio is selected to beless than stoichiometric in order to discourage the formation of carbonblack soot which would deactivate the catalyst. The rate of increase ofthe methane supply up to the steady-state value is therefore accuratelycontrolled so that no soot formation occurs.

In the case of conventionally prepared catalyst coatings, this start-upphase can take several days, especially with high reactant flows. In thecase of the catalyst coating prepared according to the invention,however, the steady state can be achieved after only a few hours,without soot formation being observed.

The coating dispersion can contain, in addition to the componentsessential to the invention, soluble and/or insoluble auxiliary agents inthe carrier liquid in order, for example, to delay sedimentation and/orto adjust the viscosity and to improve adhesion of the coating to themoulded items before and/or after forming the catalyst.

These auxiliary agents may be polymeric organic lacquer binders whichare degraded during the catalyst forming procedure without leaving aresidue such as, for example, polyacrylates, polyester resins,polyurethanes. Conventional auxiliary agents used in the production oflacquers such as organic and/or inorganic flow control agents,sedimentation retarders and thixotropic agents, such as for examplepyrogenic silica or silanes, may also be used here in effective amounts.

Furthermore, the coating dispersion may contain the adhesive oxides orprecursors for the same which are described in DE 39 23 034 C2. Theseare metal compounds in the form of oxides and/or silicates and/orborates which are capable of forming a glass at below 1000° C. and glazefrits with a hemisphere temperature below 1000° C. These substances canbe beneficial to the efficacy and operating lifetime of thecatalytically active coating. They are preferably used in an amountwhich is less than that of the nitride, preferably in an amount of 5 to50 wt. %, with reference to the nitride. A suitable adhesive oxide is,for example, magnesium oxide.

Another precursor of an adhesive oxide which may be used is an organicsilicon compound from the group of orthosilicates, organosilanes withone to three hydrolyzable groups on silicon atoms, in particulartrialkoxysilanes, or condensation products of the monomeric siliconcompounds mentioned, in particular poly(diorganosiloxanes) andpoly(organoalkoxy-siloxanes).

The process may be used to coat a variety of shaped moulded bodies suchas spheres, pellets, sponge-like structures, monoliths or tubes.Reaction tubes and monolithic honeycomb structures with parallel flowchannels are particularly preferably used, however, in the BMA process.With these moulded items, the catalytically active coating is located onthe internal walls of the tubes or flow channels. The moulded items mustbe gas-tight, that is they must possess no open porous structures, andmay consist of any known ceramic materials. Moulded items which consistsubstantially of α-aluminum oxide and may also contain, apart fromaluminum oxide, small amounts of other oxides as a result of theproduction process, are preferred.

The actual coating procedure is performed in a manner known per se,manually or using suitable coating devices, by impregnating or wettingthe surface to be coated with the dispersion and evaporating the carrierliquid. The coating apparatus described in U.S. Pat. No. 4,415,485, forexample, is suitable for the present process. After removing the excessdispersion, the solvent is removed by evaporation, for example byheating the moulded items, by flushing out with a gas and/or by reducingthe pressure. Previously heated moulded items may also be placed incontact with the dispersion.

After completion of the coating procedure, the moulded items are slowly,that is to say over the course of several hours, heated to thetemperature conventionally used for the BMA process in the BMA reactorin the presence of nitrogen, or preferably ammonia, or mixtures of thesegases.

The invention is explained in more detail by means of the examples. Theinternal walls of tubes made of α-aluminum oxide were coated withcatalytic coatings in accordance with the process according to theinvention and in accordance with conventional processes. Coating wasperformed manually by immersing the tubes in the coating dispersion.

The tubes had an internal diameter of 16 mm. For laboratory tests, tubeswith lengths of 0.5 m were used, these being heated to a maximumtemperature of 1210° C. in an electrical tubular furnace to form thecatalyst and to synthesize hydrogen cyanide. In parallel with this,production tubes of 2.1 m length were coated and formed in a productionreactor heated with hot gases and the catalytic activity was theninvestigated.

The catalyst coatings in the laboratory and production tubes were eachformed in 20 hours. For this purpose, the tubes were heated from roomtemperature to the maximum temperature over the course of 12 hours andthen held at this temperature for a further 8 hours.

COMPARISON EXAMPLE 1

A conventional coating dispersion consisting of aluminum and platinumpowder with an atomic ratio Al:Pt of 10:1 was prepared in the same wayas described in DE 39 23 034 C2.

For this, 105 g of a 40% strength Degalan® solution and 2.7 g ofAerosil® (flame hydrolytically prepared silica), 60 g ofphenyl-ethyl-polysiloxane (PEPS), were dispersed in 240 ml of toluene.166 g of aluminum powder (average particle size<60 μm) were added tothis dispersion and dispersed. The required amount of 120 g of platinumpowder (average particle size<60 μm, purity>98.5%) was dispersed in afurther 160 ml of toluene and added in portions to thealuminum-containing dispersion. Dispersion was then continued foranother 5 minutes. The dispersion obtained in this way can be stored forseveral weeks without any danger of separating. Degalan® and Aerosil®are trademarks of Degussa A. G.

This dispersion was used to coat the internal walls of a 0.5 m long tubeby immersion. 3.5 g of dispersion (dry fraction) could be deposited onthe internal wall of the tube by means of a single coating procedure. Toform the catalyst, the tube was heated from room temperature to 1210° C.over 12 hours in an electrically heated tubular furnace and held at thistemperature for a further 8 hours. A stream of ammonia flowed throughthe tube at a rate of 3 mol/h for the entire period.

After completion of forming, the synthesis of hydrogen cyanide wasstarted. For this purpose, methane was added at a slowly increasing rateof mass-flow in order to prevent the formation of soot and thusdeactivation of the catalyst. Only after four days did the methane flowcorrespond to the steady-state value of 2.7 mol/h, corresponding to amolar ratio of methane to ammonia of 0.9.

Under steady-state operation, the yield of hydrogen cyanide, withreference to methane, was 78%, so 56.9 g per hour of hydrogen cyanidecould be prepared using the tube.

EXAMPLE 1

A coating dispersion was prepared from aluminum nitride powder andplatinum powder with an atomic ratio of aluminum to platinum of 10:1.The preparation procedure was the same as that described in comparisonexample 1. Instead of 166 g of aluminum powder, however, 252 g ofaluminum nitride powder (average particle size<60 μm; purity>98%) wereused.

This coating dispersion was used to coat a 0.5 m long tube consisting ofa-aluminum oxide with a catalyst, to form the catalyst and to check itscatalytic activity. The amount of coating material was 3.2 g of drysubstance.

Forming and testing were performed in precisely the same way as incomparison example 1. The addition of methane, however, was completedafter 6 hours, without any soot formation being observed.

Under steady-state operation (3 mol/h of ammonia; 2.7 mol/h of methane,1210° C.), the yield of hydrogen cyanide, with reference to methane, was82%, so 59.8 g per hour of hydrogen cyanide could be produced with thetube.

EXAMPLE 2

Example 1 was repeated. However, the aluminum nitride powder wasreplaced by 154 g of boron nitride (atomic ratio boron to platinum:10:1) and the amount of toluene used was increased to a total of 620 ml.The boron nitride had an average particle size of <10 μm and a purityof >95%. The amount of coating material on the internal wall of the tubewas 3.7 g of dry substance.

After forming, the addition of methane could be completed within 4hours. Under steady-state operation (3 mol/h of ammonia; 2.7 mol/h ofmethane, 1210° C.), the yield of hydrogen cyanide, with reference tomethane, was 67%, so 48.8 g per hour of hydrogen cyanide could beproduced with the tube.

EXAMPLE 3

Example 1 was repeated. However, the aluminum nitride powder wasreplaced by 382 g of titanium nitride (atomic ratio titanium toplatinum: 10:1) and the amount of toluene used was increased to 590 ml.The titanium nitride had an average particle size of <10 μm and a purityof >95%. The amount of coating material was 3.4 g of dry substance.

After forming, the addition of methane could be completed within 5hours. Under steady-state operation (3 mol/h of ammonia; 2.7 mol/h ofmethane; 1210° C.), the yield of hydrogen cyanide was 52%, withreference to methane, so 37.1 g per hour of hydrogen cyanide could beproduced with the tube.

EXAMPLE 4

Example 1 was repeated. However, the aluminum nitride powder wasreplaced by 282 g of silicon nitride (atomic ratio silicon to platinum:10:1) and the amount of toluene used was increased to 590 ml. Thesilicon nitride had an average particle size of <25 μm and a purityof >90%. The amount of coating material was 2.9 g of dry substance.

After forming, the addition of methane could be completed within 7hours. Under steady-state operation (3 mol/h of ammonia; 2.7 mol/h ofmethane; 1210° C.), the yield of hydrogen cyanide was 75%, withreference to methane, so 54.7 g per hour of hydrogen cyanide could beproduced with the tube.

COMPARISON EXAMPLE 2

A 2.1 m long ceramic tube was coated with the conventional coatingdispersion from comparison example 1. After evaporating the carrierliquid, the catalyst was formed in a furnace heated by hot gases. Here,32 mol/h of ammonia flowed through the tube and the temperature wasraised from room temperature to 1320° C. over the course of 12 hours andheld at this level for a further 8 hours. The methane supply was thenintroduced at a rate of up to 29.5 mol/h of methane (molar ratio ofmethane to ammonia: 0.92). In order to avoid soot formation, the finalmethane loading of the tube could be achieved only after 9 days.

Under steady-state operation (32 mol/h of ammonia; 29.5 mol/h ofmethane, 1320° C.), the yield of hydrogen cyanide, with reference tomethane, was 78%, so 725.8 g per hour of hydrogen cyanide could beproduced with the tube.

EXAMPLE 5

A 2.1 m length ceramic tube was coated with the coating dispersion fromexample 1 and formed as described in comparison example 2. The supply ofmethane at a rate of up to 29.5 mol/h of methane could be completedafter 7 hours, without soot formation taking place.

The yield of hydrogen cyanide, with reference to methane, understeady-state operation (32 mol/h of ammonia; 29.5 mol/h of methane,1320° C.) was 86%, so 743.0 g per hour of hydrogen cyanide could beproduced with the tube.

EXAMPLE 6

Example 5 was repeated. Differently from example 5, however, forming wasperformed at a maximum temperature of only 1270° C. The methane supplycould be completed after 6 hours.

Under steady-state operation (32 mol/h of ammonia; 29.5 mol/h ofmethane, 1270° C.), the yield of hydrogen cyanide was 89%, so 769 g perhour of hydrogen cyanide could be produced with the tube. This is 6%more than in comparison example 2, although the reaction temperature is50° C. below the reaction temperature in comparison example 2.

The preceding examples show that catalysts prepared according to theinvention, after forming under high load, could be loaded with themethane stream for steady-state operation much more rapidly thanconventionally prepared catalysts. In the case of the 2.1 m longproduction tube, the supply of methane to catalysts prepared accordingto the invention could be completed in less than one thirtieth of thetime required for conventionally prepared catalysts. In addition, higheryields are obtained at lower reaction temperatures.

Conventionally prepared catalysts frequently exhibit reduced catalyticactivity after interruptions in production caused by furnace problems.This could not be observed with catalysts prepared according to theinvention. After interruptions in production, they very rapidly achievedtheir original level of activity again.

Further variations and modifications of the foregoing will be apparentto those skilled in the art and are intended to be encompassed by theclaims appealed hereto.

German priority application 196 17 040.0 is relied on and incorporatedherein by reference.

We claim:
 1. A process for preparing catalytically active coatings onthe surfaces of a shaped ceramic moulded body for the synthesis ofhydrogen cyanide from hydrocarbons and ammonia comprising uniformlywetting a ceramic moulded body with a coating dispersion which containsat least one particulate elemental platinum group metal and aparticulate nitride of an element selected from the group consisting ofaluminum, boron, titanium, silicon and mixtures thereof as cation in avaporizable carrier liquid, evaporating the carrier liquid andconverting the coating into the catalytically active state by slowlyheating to 1000 to 1350° C. in the presence of nitrogen and/or ammonia,wherein said particulate nitride and said particulate platinum groupmetal have an average particle size of substantially less than 100 μm.2. The process according to claim 1, wherein the atomic ratio ofplatinum group metal to cation is selected to be within the range from0.001 to
 1. 3. The process according to claim 1, wherein an inertorganic solvent or a solvent mixtures is the carrier liquid.
 4. Theprocess according to claim 3 wherein said organic solvent is a memberselected from the group consisting of aliphatic or aromatichydrocarbons, esters, ketones, ethers, alcohols and mixtures thereof. 5.The process according to claim 1, wherein said dispersion containsplatinum as a platinum group metal and the atomic ratio of platinum tocation is 0.01 to 0.5.
 6. The process according to claim 5 wherein saidratio is 0.01 to 0.2.
 7. The process according to claim 1 wherein saiddispersion contains 10 to 300 wt. %, of platinum powder and nitridepowder, with reference to the carrier liquid.
 8. The process accordingto claim 1 wherein said dispersion contains 30 to 200 wt. %, of platinumpowder and nitride powder, with reference to the carrier liquid.
 9. Theprocess according to claim 1 wherein said dispersion contains 50 to 150wt. %, of platinum powder and nitride powder, with reference to thecarrier liquid.
 10. The process according to claim 1 wherein thedispersed platinum group metal and nitride particle have a particle sizeof less than 50 μm.
 11. The process according to claim 1 wherein saiddispersion contains adhesive oxides or precursors of adhesive oxides.12. The process according to claim 11 wherein said dispersion contains,as precursor of an adhesive oxide, an organic silicon compound selectedfrom the group consisting of orthosilicates, organosilanes with one tothree hydrolyzable groups on silicon atoms, and condensation products ofthe monomeric silicon compounds.
 13. The process according to claim 12wherein the organic silicon compound is a trialkoxysilane.
 14. Theprocess according to claim 12 wherein said organic silicon compound is apoly(diorganosiloxane) or a poly(organo-alkoxysiloxane).
 15. The processaccording to claim 1 wherein said dispersion contains a metal compound,in a composition capable of forming a glass below 1000° C. and/or glazefrits with a hemisphere point below 1000° C.
 16. The process accordingto claim 15 wherein said metal compound is a member selected from thegroup consisting of an oxide, silicate, borate and mixtures thereof. 17.The process according to claim 15 wherein a glaze frit is used.
 18. Theprocess according to claim 9, wherein metal compound and/or glaze fritare present in an amount which is less than that of the nitride.
 19. Theprocess according to claim 18 wherein said metal compound and/or glazefrit are present in an amount of 5 to 50 wt. % of the nitride.
 20. Theprocess according to claim 1, wherein the dispersion also containsorganic polymeric binders.
 21. The process according to claim 1 whereinthe amount of platinum group metal applied to the surface of the mouldeditem with the dispersion leads to an area concentration of less than 10mg/cm².
 22. The process according to claim 21 wherein said concentrationis 0.1 to 5 mg/cm².
 23. The process according to claim 22 wherein saidconcentration is 0.5 to 2 mg/cm².
 24. The process according to claim 1wherein said heating is carried out in 2 to 20 hours to reach a BMAreaction temperature and thereafter subjecting said body to saidreaction temperature for an additional 2 to 10 hours.